Re: libxl / xen-pciback interaction [and 1 more messages]

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.03.2021 16:12, Ian Jackson wrote:
> Jan Beulich writes ("libxl / xen-pciback interaction"):
>> while trying to test a pciback fix for how SR-IOV VFs get placed in the
>> guest PCI topology I noticed that my test VM would only ever see the 1st
>> out of 3 VFs that I passed to it. As it turns out, the driver watches
>> its own (backend) node, and upon first receiving notification it
>> evaluates state found in xenstore to set up the backend device.
>> Subsequently it switches the device to Initialised. After this switching,
>> not further instances of the watch triggering would do anything.
> 
> This makes it sound like this driver does not support hotplug.
> 
>> While doing this it also occurred to me as odd how "num_devs" is
>> actually used: It's not really a "number of devices" indicator, but
>> instead a "next device has this number" one: libxl reads the present
>> value and increments it by one for every new device. Destroying
>> (hot-unplugging) of devices doesn't have any effect on the value.
> 
> But this makes it sound like the driver *does* support hotplug.
> 
> How does what libxl is doing differ from a setup, immediately followed
> by a hot-add ?

In the hot-add case libxl drives things through Reconfiguring state.
I'm not sure this would be an appropriate (and backwards compatible)
thing to do when initially populating xenstore.

Jan